AMP works great in Firefox (modulo this bug right now https://github.com/ampproject/amphtml/issues/5479 :) and all other modernish browsers. Google Search currently chooses not to display it for FX, but that is not due to a limitation in AMP.
There’s been discussion about adding Google Search to the list of sites for which Firefox Mobile always serves a faked User Agent because of this hostile and anticompetitive behaviour before.